The current structure of defence management in Britain was set in place in 1964 when the modern day Ministry of Defence (MoD) was created (an earlier form had existed since 1940). The MoD assumed the roles of the Admiralty, the War Office and the Air Ministry.

Post–World War II economic and political decline, as well as changing attitudes in British society and government, were reflected by the Armed Forces' contracting global role.[12][13][14] Britain's protracted decline was dramatically epitomised by its political defeat during the Suez War of 1956.[15] The 1957 Defence White Paper abolished conscription and reduced the size of the Armed Forces from 690,000 to 375,000 by 1962.[16] Seeking an inexpensive alternative to maintaining a large conventional military, the government pursued a doctrine of nuclear deterrence.[17][18] This initially consisted of free-fall bombs operated by the RAF, but these were eventually superseded by the submarine-launched Polaris ballistic missile. While assurances had been made to the United States that Britain would maintain a presence "East of Suez", a process of gradual withdrawal from its eastern commitments was undertaken in the 1960s, primarily for economic reasons.[19][20] By the mid-1970s, Britain had withdrawn permanently deployed forces from Aden, Bahrain, Malaysia, Mauritius, Oman, Sharjah, and Singapore. Agreements with Malta (expired 1979) and South Africa (terminated 1975) also ended.

The Vulcan Bomber was the backbone of the United Kingdom’s airborne nuclear deterrent during much of the Cold War.

With a permanent presence east of Suez effectively reduced to Hong Kong (up to 1997) and Brunei, the Armed Forces reconfigured to focus on the responsibilities allocated to the services during the Cold War.[14][21][22] Substantial forces thus became committed to NATO in Europe and elsewhere; by 1985, 72,929 personnel were stationed in Continental Europe.[22][23][24] The British Army of the Rhine and RAF Germany consequently represented the largest and most important overseas commitments that the British Armed Forces had during this period.[25] The Royal Navy's fleet developed an anti-submarine warfare specialisation, with a particular focus on countering Soviet submarines in the Eastern Atlantic and North Sea.[21] In the process of this transition and due to economic constraints, four conventional aircraft carriers and two "commando" carriers were decommissioned between 1967 and 1984.[26][27] With the cancellation of the CVA-01 project, three Invincible-classSTOVLaircraft carriers, originally designed as "Through-Deck Cruisers", became their ultimate replacements.[27]

While this focus on NATO obligations increased in prominence during the 1970s, low-intensity conflicts in Northern Ireland and Oman emerged as the primary operational concerns of the British Armed Forces.[28] These conflicts had followed a spate of insurgencies against British colonial occupation in Aden, Cyprus, Kenya and Malaysia.[28] An undeclared war with Indonesia had also occurred in Borneo during the 1960s, and recurring civil unrest in the declining number of British colonies often required military assistance.

Four major reviews of the British Armed Forces have been conducted since the end of the Cold War. All three services experienced considerable reductions in manpower, equipment, and infrastructure during this period[29] while re-structuring to deliver a greater focus on expeditionary warfare.

An increasingly international role for the British Armed Forces has been pursued since the Cold War's end.[33] This entailed the Armed Forces often constituting a major component in peacekeeping missions under the auspices of the United Nations or NATO, and other multinational operations. Consistent under-manning and the reduced size of the Armed Forces highlighted the problem of 'overstretch' during the conflicts in Iraq and Afghanistan.[34] This reportedly contributed to personnel retention difficulties and challenged the military's ability to sustain its overseas commitments.[34][35][36]

The first major post-11 September restructuring was announced in the 2004 Delivering Security in a Changing World: Future Capabilities review, continuing a vision of "mobility" and "expeditionary warfare" articulated in the SDR.[42][43] Future equipment projects reflecting this direction featured in the review, including the procurement of two large aircraft carriers and a series of medium-sized vehicles for the Army. Reductions in manpower, equipment, and infrastructure were also announced. The decision to reduce the Army's regular infantry to 36 battalions (from 40) and amalgamate the remaining single-battalion regiments was controversial, especially in Scotland and among former soldiers.[44] Envisaging a rebalanced composition of more rapidly deployable light and medium forces, the review announced that a regiment of Challenger 2main battle tanks and a regiment of AS-90 self-propelled artillery would be converted to lighter roles.[43][45]

There were more than 30,000 members of the British Armed Forces deployed abroad in January 2007, serving in various capacities.[46][47] Peacekeeping, humanitarian aid, and disaster relief tasks increased in the 2000s, many under the auspices of the United Nations and NATO.[48] The Armed Forces contributed to the international humanitarian and reconstruction efforts in the aftermath of the 2004 tsunami and 2005 earthquake in Pakistan.[49][50]

The British Armed Forces is a professional force with a strength in January 2015 of 156,940 Regular[2] and 30,000 Volunteer Reserve personnel.[2] This gives a total strength of 186,940 Service Personnel.[2] In addition, all ex-Regular personnel retain a "statutory liability for service" and are liable to be recalled (under Section 52 of the Reserve Forces Act (RFA) 1996) for duty in a time of need, this is known as the Regular Reserve. However, MoD publications since April 2013 no longer report the entire strength of the Regular Reserve, instead they only give a figure for Regular Reserves who serve under a fixed-term reserve contract. These contracts are similar in nature to those of the Volunteer Reserve.[4] As of 2014, regular Reserves serving under a fixed-term contract numbered 45,110 personnel in 2014.[3] All personnel figures exclude the Military Provost Guard Service and the University Training Units; the University Royal Naval Unit, the Officers' Training Corps and the University Air Squadron.

Britain has the fifth or sixth-largest defence budget in the world,[56] with the country spending more than countries like Germany or Japan but more or less comparable to that of France or Saudi Arabia. In September 2011, according to the Royal United Services Institute, current "planned levels of defence spending should be enough for the United Kingdom to maintain its position as one of the world's top military powers, as well as being one of NATO-Europe's top military powers. Its edge – not least its qualitative edge – in relation to rising Asian powers seems set to erode, but will remain significant well into the 2020’s, and possibly beyond."[57]

In the 2013 Spending Review, the Chancellor of the Exchequer, George Osborne, reinforced the government's commitment to the 2010 SDSR and stated the £38bn "black hole" in the defence budget had been filled, that at over 2% of GDP, the defence budget will remain one of the largest in the world, the equipment budget will grow by 1% in real terms every year after 2015, that there will be further reductions in the civilian MoD workforce, PFI contracts signed in the last decade would be renegotiated, the way equipment is purchased is to be overhauled, and a greater commitment to cyber warfare.[58]

The Royal Air Force established an air transport and refuelling hub at Al-Minhad in the United Arab Emirates in 2013. The following year, the Foreign and Commonwealth Office announced it would expand the UK's naval facilities in Bahrain to support larger Royal Navy ships deployed to the Persian Gulf. Once complete, it will be the UK's first permanent military base located East of Suez since it withdrew from the region in 1971. The base will reportedly be large enough to accommodate Type 45 destroyers and Queen Elizabeth class aircraft carriers.[59][60][61]

The Ministry of Defence is the Government department and highest level of military headquarters charged with formulating and executing defence policy for the Armed Forces; it currently employs over 60,000 civilians as of 2014. This number will be reduced to just 55,000 by 2015 (a reduction of 25,000 as per the October 2010 SDSR) and then again to 48,000 by 2020 (a reduction of 7,000 as per the Three Month Review in 2011).[64] The department is controlled by the Secretary of State for Defence and contains three deputy appointments: Minister of State for the Armed Forces, Minister for Defence Procurement, and Minister for Veterans' Affairs.

Responsibility for the management of the forces is delegated to a number of committees: the Defence Council, Chiefs of Staff Committee, Defence Management Board and three single-service boards. The Defence Council, composed of senior representatives of the services and the Ministry of Defence, provides the "formal legal basis for the conduct of defence". The three constituent single-service committees (Admiralty Board, Army Board and Air Force Board) are chaired by the Secretary of State for Defence.

A successor programme is currently in its early stages with a final decision to be made in 2016 after the 2015 general election. It primarily seeks to replace the Vangaurd-class submarines with a new generation of SSBNs, however, the programme will also extend the life of the UGM-133 Trident II ballistic missiles, refurbish the nuclear warheads and modernise existing infrastructure associated with the deterrent.[69]

Former weapons of mass destruction possessed by the United Kingdom include both biological and chemical weapons. These were renounced in 1956 and subsequently destroyed.

The UK Joint Expeditionary Force, not to be confused with the similarly named UK-French Combined Joint Expeditionary Force, was announced in December 2012 by the Chief of the Defence Staff, General Sir David Richards.[70] It is designed to be an integrated joint force, with capabilities across the spectrum at sea, on land and in the air, with the aspiration being greater levels of integration than previously achieved especially when combined with other nations' armed forces. Of variable size, it is intended to be the basis of all the UK armed forces' combined joint training; a framework into which other nations will fit. It will be the core of the UK's contribution to any military action, whether NATO, coalition or independent. Together with Command and Control elements including HQ Allied Rapid Reaction Corps and the maritime component HQ at Northwood, the force is designed to meet the UK's obligations to NATO.

A submarine service has existed within the Royal Navy for more than 100 years. The Submarine Service's four Vanguard-class nuclear-powered submarines carry Lockheed Martin's Trident II ballistic missiles, forming the United Kingdom's nuclear deterrent. The service possessed a combined fleet of diesel-electric and nuclear-powered submarines until the early 1990s. Following the Options for Change defence review, the Upholder class diesel-electric submarines were withdrawn and the attack submarine flotilla is now exclusively nuclear-powered. Seven Astute class nuclear-powered attack submarines have been ordered, with two completed, and four under construction. The Astute class are the most advanced and largest fleet submarines ever built for the Royal Navy, and will maintain Britain's nuclear-powered submarine fleet capabilities for decades to come.

The British Army is made up of the Regular Army and the Army Reserve. The army has a single command structure based at Andover and known as "Army Headquarters".[79][80] Deployable combat formations consist of two divisions (1st Armoured and 3rd Mechanised) and eight brigades.[81][82] Within the United Kingdom, operational and non-deployable units are administered by three regionally defined "regenerative" divisions (2nd, 4th, and 5th) and London District.[83]

The Royal Air Forces operates multi-role and single-role fighters, reconnaissance and patrol aircraft, tankers, transports, helicopters, unmanned aerial vehicles, and various types of training aircraft.[91] Ground units are also maintained by the Royal Air Force, most prominently the RAF Police and the Royal Air Force Regiment (RAF Regt). The Royal Air Force Regiment essentially functions as the local ground defence force of the RAF.[92] Roled principally as ground defence for RAF facilities, the regiment contains nine regular squadrons, supported by five squadrons of the Royal Auxiliary Air Force Regiment. By March 2008, the three remaining "Air Defence" squadrons had disbanded or re-roled and their responsibilities transferred to the British Army's Royal Artillery.[93]

On the 21st July 2015, the British government disclosed that by 2019, the UK's frontline fighter force will lose 140 aircraft, leaving only 127 combat-effective fighters, the lowest number since the RAF was created. According to Janes Defence this will mean a reduction of 77% as the simultaneous retirement of the Tornado GR4 and Tranche 1 Typhoon's are affected. This will be slightly offset by the introduction of Tranche 3 Typhoon's and early F35B's, however, by 2019 the UK will have only 15 to 20 F35B's which will not be fully combat-ready until 2023. As the UK has only ordered 14 F35B's and look to perhaps only purchase 48 of the original 138 this will leave the UK fielding the lowest number of fighter aircraft in it's history. [94]

The British Armed Forces are supported by civilian agencies owned by the MoD. Although they are civilian, they play a vital role in supporting Armed Forces operations, and in certain circumstances are under military discipline.

The 13 ships of the Royal Fleet Auxiliary (RFA) primarily serve to replenish Royal Navy warships at sea, and also augment the Royal Navy's amphibious warfare capabilities through its three Bay-classlanding ship dock vessels. It is manned by 1,850 civilian personnel and is funded and run by the Ministry of Defence.

One of the most recognisable recruiting posters of the British Army; from World War I featuring Kitchener.

All three services of the British Armed Forces recruit primarily from within the United Kingdom, although citizens from the Commonwealth of Nations and the Republic of Ireland are equally eligible to join.[95] The minimum recruitment age is 16 years (although personnel may not serve on armed operations below 18 years, and if under 18 must also have parental consent to join); the maximum recruitment age depends whether the application is for a regular or reserve role; there are further variations in age limit for different corps/regiments. The normal term of engagement is 22 years; however, the minimum service required before resignation is 4 years, plus, in the case of the Army, any service person below the age of 18.[96] At present, the yearly intake into the armed forces is 11,880 (per the 12 months to 31 March 2014).[97]

Excluding the Brigade of Gurkhas and the Royal Irish Regiment, as of 1 April 2014 there are approximately 11,200 Black and Minority Ethnic (BME) persons serving as Regulars across the three service branches - of those, 6,610 were recruited from outside the United Kingdom. In total, Black and Minority Ethnic persons represent 7.1% of all service personnel, an increase from 6.6% in 2010.[97]

Women have been integrated into the British Armed Forces since the early 1990s; however, they remain excluded from primarily combat units in the Army, Royal Marines, and Royal Air Force Regiment.[101] As of 1 April 2014, there are approximately 15,840 women serving in the Armed Forces, representing 9.9% of all service personnel.[97] The first female military pilot was Flight Lieutenant Julie Ann Gibson while Flight LieutenantsJo Salter and Kirsty Moore were the first fast-jet pilots, the former flying a Tornado GR1 on missions patrolling the then Northern Iraqi No-Fly Zone.[102] Flight Lieutenant Juliette Fleming and Squadron Leader Nikki Thomas recently were the first Tornado GR4 crew.[103] While enforcing the Libyan No-Fly Zone, Flight Lieutenant Helen Seymour was identified as the first female Eurofighter Typhoon pilot.[104] In August 2011, it was announced that a female Lieutenant Commander, Sarah West, was to command the frigateHMS Portland.[105]

^Figure is current as of 1 January 2015. It includes 30,000 Volunteer Reserve personnel[2] and 45,110 Regular Reserve personnel.[3] Since April 2013, MoD publications no longer report the entire strength of the Regular Reserve, instead, only Regular Reserves serving under a fixed-term reserve contract are counted. These contracts are similar in nature to the Volunteer Reserve.[4] The figure for Volunteer Reserve personnel excludes the University Training Units.

^Also referred to as Her Majesty's Armed Forces, the Armed Forces of the Crown, and the Armed Forces of the United Kingdom.